Many silicone elastomers, when unfilled, generally have excellent optical clarity and are usually water-white in color. The tensile strength, and especially the tear strength, of such elastomers is poor, however. By filling the elastomers with fine particulate solids, the silicone resin or gum stock will produce, when vulcanized and cured, an elastomer with much improved strength. Fillers used with silicone elastomers have been the usual fillers for plastics, but the fillers are limited when optical clarity of the elastomer is essential. Commonly used dimethyl siloxane, with a vulcanizing agent, produces a silicone elastomer, but for satisfactory use it needs a filler for strength. One filler used for silicone elastomers is finely divided silica, known as smoke or fume silica. This filler when added to dimethyl siloxane elastomer, or gum stock and then vulcanized, produces a translucent elastomer, generally considered useless for objects needing optical clarity, such as lenses. This is primarily due to the mismatch of the indices of refraction of fume silica and dimethyl siloxane.
In U.S. Pat. No. 3,341,490 there is described a blend of vinyl-type siloxane units which may be filled with a silica filler forming, after vulcanizing and curing, products which are useful in the manufacture of articles having optical clarity.